Cellular telecommunications systems operating according to some form or variation of the European Telecommunications Standards Institute (ETSI) Global System for Mobile Communications (GSM) standard are prevalent worldwide. Those systems include standard GSM systems operating in the 900 MHz frequency band, personal communications systems (PCS) operating in the 1900 MHz frequency band, and Digital Communication Systems (DCS) operating in the 1800 MHz frequency band.
In a system operating according to the GSM standard, when a mobile station operating in idle mode loses service and can no longer locate an acceptable serving cell using the GSM cell reselection procedure within the serving network, a process is initiated whereby the mobile station begins to search for a new serving cell or serving network on a broader basis. The new serving cell that is sought may be in the network from which service was lost or in a separate network. The search procedure used upon loss of service is the same procedure used for establishing service initially. This procedure includes the public land mobile network (PLMN) and cell selection procedures specified in Section 6 of the ETSI document GSM 05.08 Version 6.20, TS 100 911 V 6. 20 (1998-07) and Sections 4.4 and 4.5 of the ETSI document GSM 03.22 Version 6.0.0 IS 100 930 V6.0.0 (1998-07).
The loss of service procedures specified for GSM assume that when loss of service occurs, a mobile station has moved away from the serving network and cells of the serving network available for reselection immediately prior to loss of service will no longer be available for reselection after loss of service. The procedure, therefore, begins to initialize service with a network search. The network search involves locating a network and searching, through the GSM cell selection process, all RF channels of the system in which the selected network is included. This involves searching up to 124 channels for GSM 900 MHz systems, 299 channels for GSM 1900 MHz (PCS 1900) systems, and up to 374 channels for GSM 1800 MHz systems (DCS 1800). It is also possible to specify a fewer number of channels to be searched in cell selection through the use of stored lists in the mobile station.
The assumption that the mobile station has moved away from the serving network, however, is not always correct, and using this loss of service procedure may cause problems or inefficiencies in mobile station operation. When loss of service is encountered, the PLMN and cell selection procedures will be initiated within the mobile station and the PLMN and/or cell selection procedures will be performed even though the mobile station may still be located in the service area of the former serving network, and strong signals are immediately again available to the mobile station from the former serving network. Continually initiating the PLMN and/or cell selection procedures could be inefficient.
For example, a mobile station may be located in a serving network where signals from all serving network cells being considered for reselection may be temporarily weakened, or blocked, as the user of a mobile station moves through certain areas of the serving network. In this type of situation, if all the signal strengths from candidate cells for reselection fall below a threshold level C1, loss of service from the serving network is assumed. The loss of service may happen over and over again. The actual loss of service also could be very short in duration if the weakening or blocking of the signals of the candidate cells happens for only a very short period of time, as, for example, if the user of a mobile station rapidly drives through a tunnel or moves rapidly among or within buildings. In this case, the signal strength of one or more candidate cells of the serving network may quickly rise again above the threshold level C1 after loss of service. However, even though these cells of the serving network are available again, the mobile station will initiate a network search. The network search will eventually bring the mobile station back to the former serving network.
Because performing the PLMN and cell selection procedures consume significant power, it is common to implement a power saving mode for PLMN and cell selection for GSM. In the power-saving mode, a mobile station typically repeats the PLMN and cell selection procedures every x seconds, rather than repeating the process continuously. X may be, for example, a value of 6 seconds. This power-saving mode could have negative effects as far as time to re-establish service in areas where network coverage is poor and the user of a mobile station is rapidly moving. In this case, a significant percentage of time may be spent performing the PLMN and/or cell selection procedures when the mobile station will only end up back on the former serving network. As the total percentage of time spent on PLMN and cell selection rises, the less time the mobile station indicates available service to the user. Also, there will be less overall time during which the user will be able to receive calls.